This invention relates in general to the constuction of a gas sensor and to a method of manufacturing a zinc oxide semi-conductor for such sensor and, in particular, to a new and useful sensor having a metallic semi-conductor arranged in a gas stream formed by a casing which is adapted to be heated and wherein electrodes of respective polarity are connected to respective ends of a metallic oxide semi-conductor which is formed of zinc oxide and then sintered at a temperature above 750.degree. C.